C.E.D. calls their ballistic measuring device a chronograph:
As does Chrony:
As for the definition of a chronometer:
As to your question of wanting to buy a bullet velocity measuring device, what ever it's called, the low priced Chronys do a credible job.
I've had one of the basic ones for decades.
As long as it's not shot, it will work just fine.
Mines been wounded a few times without damage.
All of them are a little finicky with light sources, though.